The present invention relates to a storage layer for storing and a conversion layer for storing and converting x-ray information which has a multitude of needle-shape storage material areas as well as a device for reading x-ray information from a storage layer and an x-ray cassette therefor.
In particular for medical purposes, an image is generated by x-radiation of an object, for example, of a patient, where the image is stored in a storage layer as a latent image. Thus, such an x-radiation image contains x-ray information about the object. To read the x-ray information stored in the storage layer, it is excited using a radiation source. Due to this excitation, the storage layer emits light that exhibits an intensity that corresponds to the x-ray information stored in the storage layer. The light emitted by the storage layer is received by a receiving means such that the x-ray information stored in the storage layer can then be made visible. For example, the x-ray information can be displayed directly on a monitor. Typically, such storage layers are applied to a carrier material that can be either transparent or reflecting. With a reflecting carrier material, both the radiation source and the receiving means are arranged on the same side of the carrier material, that is, on that side of the carrier material where the storage layer is located. If the storage material is located on a transparent carrier material, then the radiation source is located on one side of the carrier material, and the receiving means on the opposite side of the carrier material. This arrangement has the particular advantage that a larger amount of the radiation emitted by the storage layer can be captured by the receiving means. A better reproduction quality of the x-ray information stored in the storage layer is, therefore, possible.
For example, from the patent document DE 198 59 747 C1 it is known to use a special storage layer for storing x-ray information, where the storage layer exhibits a special crystallite, needle-shaped structure. The special storage layer exhibits numerous “needles” that can serve the purpose of guiding both the excitation and the emission radiation. Crystalline “needles” are cultivated for such storage layers. Such a needle storage layer is constructed of binary alkali halides, such as cesium bromides, CsBr. These structured alkali halides can be doped with suitable activators such as gallium, thallium, europium, etc. Depending on their given purpose, the individual needle crystals vary in height between 100 and 600 μm and have a thickness of about 10 μm. Typically, the individual needles are separated from one another by a small air gap. Both the excitation and the emission lights are guided in the individual needles that serve as light conductors according to the principle of total reflection. Incident excitation radiation that arrives at a certain angle is largely transmitted without scattering until it strikes an information center in the crystal lattice of the needle, where the x-ray information is stored. The emission radiation that is generated through the excitation of the information center is transferred in the respective needle and is guided out of this needle such that it can be detected by the receiving means. Such a needle-shaped storage layer is known, in particular, from the European patent application EP 0 751 200 A1. Using this special storage layer reduces scattering of the excitation radiation within the storage layer. In particular, when reading the x-ray information that is stored in the storage layer line by line, scattering of the excitation radiation perpendicular to the direction of the lines is disadvantageous because information centers may be excited that belong to a line of the storage layer other than the one that is being read at the moment. In this manner, emission radiation may get “lost”; i.e., it cannot be detected by the receiving means. In addition, scattering of the emission radiation within the storage layer is reduced with the result that in particular a good local resolution is achieved for the detection of the emission radiation in the receiving means. However, it has been found that, for example, excitation radiation that enters the storage layer at an incident angle that is greater than a certain angle does not remain in the respective needles but instead passes perpendicular through these needles. Especially because these needles exhibit an irregular outer structure, scattering of the excitation radiation can occur that is disadvantageous for the quality of the reproduction of the x-ray information. Since, in particular, the irregular outer structure of the needles results in a portion of the excitation radiation not being fully reflected in the needle, a blurring is created in the reproduction of the x-ray information. A similar situation applies to the emission radiation that is essentially emitted isotropically by an information center that is struck by the excitation radiation. Due to the aperture angle that is determined by the relation between the refractive index of air to the alkali halide of which the individual needles have been cultivated, a portion of the emission light is not fully reflected in the needle but instead is emitted from the respective needle. This leads to a corresponding degradation of the local resolution when detecting the emission radiation.
Alternatively to the interim storing of x-ray information in the storage layer, as described above, x-ray information that is contained in the x-radiation can also be converted directly into light radiation using a conversion layer. This light radiation that contains an image of the x-ray information can then be detected by a light-sensitive sensor and converted into electrical signals. Such a conversion layer and a device where such a coating is used are known, for example, from the patent documents DE 195 05 729 C1, DE 195 06 809 A1 or DE 195 09 021 C2. The conversion layer for converting the x-radiation into light radiation is designated as a so-called scintillator layer that may consist essentially of cesium iodide CsI. X-ray detectors that contain such conversion layers are already available on the market today. For example, the company Trixell, 460 Rue de Pommarin, 38430 Moirans, France, uses such a conversion layer in their product Pixium 4600. These conversion layers for converting x-radiation into light radiation contain numerous conversion zones with materials that directly convert x-radiation into light radiation. Similar to the storage layers described above, these conversion zones are arranged in the conversion layers in needle-shape next to one another. This means that the conversion of x-radiation to light radiation occurs in the individual needles. The light energy, which has a low energy in comparison to the x-radiation, can exit a needle where it was generated due to the aperture angle at the barrier layers of the needles and can arrive at one or more other needles. This has the result that light radiation that has been generated in a certain needle exits that conversion layer at an entirely different location, and is therefore detected by the light-sensitive sensor at a location that does not correspond to the location of the needle where the light radiation was generated. As was the case previously with the storage layers, the local resolution is falsified during the detection of the light radiation that is emitted by the conversion layer due to the described scattering.